Adding a crusader subclass for warriors to take advantage of followers might help make religion a bit more used though 
Although i like the idea, i am afraid that making features like religion to useful, or worse, appear necessary in order to wage a successful war, will in the end ruin the feature, turning it more into a tool then a great way to add flavour and dimension to the game.
As long as a feature like crusader class doesn't impact combat too significantly it can add to the religion game in a positive way.
I personally think religion needs improvement in the way people use them, the mechanics are quite good and statisfying.
It's just the lack of a good story and player involvement that gives the idea that religion needs improvement.
SA had quite a good structure and story but less people invest time playing the religion game.
